The present invention is directed to a microwave oven for heating food and comprising an oven cavity and a microwave source for generating microwaves in the cavity, and more particularly to a control device for controlling the microwave power in the oven cavity.
Power controlled microwave ovens have been implemented in different ways. GB-A2-108 734 describes a device in which the power supplied to the microwave source, normally a magnetron, is measured and dependent on the measured value the number of mains voltage cycles that are supplied to the power source is varied in order to obtain the desired power. U.S. Pat. No. 4,420,668 is likewise based on power measurement and control of the magnetron operating time dependent thereon. EP-A2-0.043.958 describes another device which is based on power measurement by measuring the current and voltage supplied to the microwave power source. SE 7510018-0 describes a device comprising a power source of the switch mode type, in which the current to the magnetron is measured and in which the measured current value is used for controlling a frequency transformation in the power source to influence the power which is supplied.
The above mentioned power control devices all are based on measurement of power or current. This means that the devices will be relatively demanding with respect to components with a consequent increase in complexity and manufacturing cost.
Another method for power control in microwave ovens is based on control of the magnetron operating time. Normally the magnetron always operates at maximum power and lower power levels are obtained by so-called pulsing of the magnetron. This means that the operating time of the magnetron is subdivided into operation periods and that the magnetron is switched on during parts of the operation period in such a manner that the mean power value during an operation period corresponds to the desired power level. For example, if a fifteen second operation period is used a mean power level corresponding to 60% of the maximum power may be obtained by switching on the magnetron for nine seconds and switching off the same for six seconds during each operation period. This time control is useful also in combination with another control mode such as power control according to the patent specifications discussed above. An example would be time control in combination with a switch mode power source having a controllable output power.
Generally, the components comprising a microwave supply system, e.g. transformer, magnetron, high voltage capacitor, exhibit component tolerances which influence the power supplied. In the case of controlling the duration of power levels which are lower than the maximum power as described above, there are also variations in the time before the magnetron starts generating microwaves, after the generation of a start signal by a control device. The variations of the starting time are influenced by the component tolerances but are in the first instance strongly dependent on the mains current which is supplied to the microwave oven. As a consequence a deviation will arise between the nominally calculated switch-on interval of the magnetron and that which is actually obtained during an operation period and consequently a corresponding deviation will occur in the average power during the operation period.